Can I claim my child if a divorce decree says we share?

What is the name of your state? Washington but divorce is filed in Idaho...ex lives in yet another state.
I want to claim both children on my taxes.
The divorce decree states we should both claim one child on taxes, yet my ex has not followed the instructions on the decree regarding several issues.
The decree was based on him paying a percent of their out of pocket medical (one child is disabled). we have received $0
There has been a total of 5 (2hr) visits in 8 years. We have joint legal, I have sole physical custody. My ex is thousands behind in child support, and even more in Medical.
Can I claim the children I am raising on my taxes?

If the divorce decree lets him claim 1 child unconditionally, you can't claim both children. If it conditions his claiming a child on his having his child support paid in full on 12/31 every year, you can claim the child. If the current decree is unconditional, you can file in WA to modify the parenting plan to give you the exemptions or to condition his right to claim a child on paying support.